The manufacturers and users of internal combustion engines have long sought an efficient and cost effective method to reduce the emissions of certain exhaust gases produced by such engines. Automobile manufacturers have addressed the problem with the incorporation of catalytic converters which promote the oxidation-reduction of undesirable gases such as hydrogen, carbon monoxide, hydrocarbons such as linear and branched alkanes, alkenes, aromatics, and nitrogen oxides, sulfurdioxide, water vapor and mixtures thereof.
In general, motor vehicle exhaust gases are passed over or through a body containing one or more substances which act as catalysts for the desired reactions. Catalysts which are able to promote the oxidation of hydrocarbons and carbon monoxide as well as the reduction of nitrogen oxides are known as Three Way Catalysts (TWC). Whether oxidation or reduction is the prevalent reaction in such catalysts is a function of the stoichiometry of the air/fuel ratio. In air-rich fuel mixtures, oxidation reactions will dominate, while fuel-rich feed mixtures encourage reduction.
As the following discussion of the prior art indicates, much effort has been directed toward the formulation of efficient, low cost catalysts which are capable of withstanding normal automobile operating conditions.
While the catalytic activity of the noble metals has long been known by those skilled in the art, several prior art patents have attempted to increase their catalytic activity and efficiency. Many of these attempts have centered on the addition of one or more metals to the noble metals to achieve multi-metallic catalysts. For example, U.S. Pat. No. 4,036,784, issued to Gembicki et al., and U.S. Pat. No. 4,324,645, issued to Angevine et al., herein incorporated by reference, both disclose catalyst compositions and/or processes of using such wherein the catalysts include combinations of Group VIB and Group VIII metals.
More particularly, U.S. Pat. No. 4,374,103, issued to Gandhi et al., and U.S. Pat. No. 4,500,650, issued to Wyatt et al., herein incorporated by reference, disclose the addition of tungsten containing compounds to respectively increase the catalytic activity of palladium and the platinum group metals.
However, under normal operating conditions, such catalysts may reach operating temperatures between 300.degree. to 1200.degree. C. Tungsten oxide from these catalysts is lost at temperatures as low as 800.degree. C. As a result, such prior art catalysts will be prematurely depleted of tungsten oxide.
Thus, although the prior art was able to produce Three Way Catalysts having increased activity, such prior art catalysts are not suitable for commercialization due to the volatility of the tungsten oxide and the corresponding depletion of such under normal operating conditions. Such problems are particularly acute in motor vehicles such as trucks which normally have catalyst operating temperatures in the 700.degree.-1000.degree. C. range.
The instant invention has addressed the deficiencies in the prior art by incorporating basic metal oxides which act to surface stabilize the tungsten oxide. Although not wishing to be bound to a particular theory, it is believed that this stabilization occurs as a result of the interaction of the basic metal oxide with the acidic tungsten oxide. In addition, it has been found that the catalysts of the instant invention have increased activity for the oxidation of hydrocarbon gases.
While various prior art patents, including U.S. Pat. Nos. 4,500,650; 4,749,671; and 4,931,419; herein incorporated by reference, disclose the addition of various metal oxides to noble metal catalysts containing tungsten, nowhere does the prior art teach that the addition of basic metal oxides will stabilize the acidic tungsten oxide and thereby decrease the volatility. The prior art has neither addressed nor solved the problem of tungsten oxide sublimation under normal catalyst operating conditions.
Accordingly, it is an object of the invention to produce a commercially feasible catalyst which is both efficient and cost effective.
It is a further object of the invention to provide an exhaust gas catalyst which has the increased catalytic activity of a tungsten modified noble metal catalyst but which is stable and nonvolatile at the operating temperatures normally encountered by an exhaust gas catalyst used in a motor vehicle.
It is another object of the invention to provide a catalyst which exhibits increased catalytic activity for the oxidation of hydrocarbons.
It is a further object of the invention to provide a process of making the catalyst of the invention.
Finally, it is an object of the invention to provide a process for reducing the emission of exhaust gases produced by an internal combustion engine utilizing the catalyst of the invention.